1. Field of the Invention
The present invention relates to solid electrolyte electrochemical cells, and more particularly relates to solid oxide fuel cells which are operable over a wide temperature range.
2. Background Information
High temperature solid oxide fuel cells (SOFC) have demonstrated the potential for high efficiency and low pollution in power generation. However, some problems remain associated with the high temperature processing and operation of such conventional cells. For example, any interaction between the lanthanum oxide-based perovskite air electrode (AE) and the zirconia-based electrolyte (EL) to form a low conducting compound at the AE/EL interface increases both cell resistance and the air electrode polarization, which can seriously affect the cell performance and result in loss of SOFC power. One technique to avoid such interaction is to apply an interfacial material at the AE/EL interface which does not increase cell resistance or increase air electrode polarization. As disclosed in U.S. Pat. No. 5,106,706 to Singh et al., which is incorporated herein by reference, the use of ceria as an AE/EL interface material has proven effective in improving high temperature SOFC performance. However, the thermal expansion match with the electrolyte and the electrical conduction characteristics of the interface modifier need further improvement.
Successful operation of SOFCs for power generation has been limited to temperatures of around 1000.degree. C. due to insufficient electrical conduction of the electrolyte and high air electrode polarization loss at lower temperatures. U.S. Pat. Nos. 4,547,437 and 4,692,274 to Isenberg et al., which are incorporated herein by reference, disclose solid oxide fuel cells operable at relatively high temperatures. Increased versatility in generator design and operation would result if the SOFCs could operate over a wider temperature range and under a wider temperature gradient. In addition to large-scale power generation, SOFCs operable at lower temperatures would be useful in additional applications such as in powering light-duty vehicles.
Conventional solid oxide fuel cells which incorporate yttria stabilized zirconia (YSZ) electrolytes have shown good performance at high temperatures of around 1000.degree. C. owing to the chemical stability and adequate ionic conduction of YSZ. However, with a decrease in operating temperature, the power losses increase, resulting from the substantial increase in the YSZ bulk ionic resistance, the electrolyte/electrode interfacial resistance and the electrode reaction polarization. The high temperature operation also sets limitations on generator materials, which can increase costs.
In order to allow SOFCs to operate at lower temperatures, the use of ultrathin film YSZ and non-zirconia based electrolytes, such as those based on ceria, have been explored. While the application of ultrathin film YSZ electrolytes of a few microns or thinner does reduce the bulk electrolyte resistance to some extent, such ultrathin films reduce the cell long-term reliability and the resistance to gas and electron leakage through the electrolyte. The use of ceria-based electrolytes requires major modifications of the other SOFC components, such as the air electrode and interconnection, to achieve a thermal expansion match. Moreover, ceria-based electrolytes demonstrate significant electronic conduction in fuel atmospheres, resulting in substantial non-power generating fuel consumption.